In recent years, electrostatic-driving-type actuators that employ micro-electromechanical systems (MEMS) have sometimes been utilized in variable capacitance devices (for example, refer to Japanese Unexamined Patent Application Publication No. 2010-280057).
FIG. 1 is a diagram for explaining an example configuration of an electrostatic-driving-type actuator 101 of the related art that is included in a variable capacitance device.
As illustrated in FIG. 1, the electrostatic-driving-type actuator 101 includes a semiconductor substrate 102, an elastic member 103, a lower driving electrode 104A, an upper driving electrode 104B, a lower capacitance electrode 105A, an upper capacitance electrode 105B and an insulating film 106. The elastic member 103 is a movable portion composed of an insulating material and has one end thereof fixed to the semiconductor substrate 102. The lower driving electrode 104A and the lower capacitance electrode 105A are formed next to each other on the semiconductor substrate 102. The insulating film 106 is formed so as to cover the lower driving electrode 104A and the lower capacitance electrode 105A. The upper driving electrode 104B is formed on the elastic member 103 so as to face the lower driving electrode 104A. The upper capacitance electrode 105B is formed on the elastic member 103 so as to face the lower capacitance electrode 105A.
This electrostatic-driving-type actuator 101 is driven by applying a driving voltage (DC voltage) between the lower driving electrode 104A and the upper driving electrode 104B. Specifically, a driving capacitance is formed between the lower driving electrode 104A and the upper driving electrode 104B as a result of the driving voltage being applied between the lower driving electrode 104A and the upper driving electrode 104B. The elastic member 103 is drawn toward the semiconductor substrate 102 by electrostatic attraction due to the driving capacitance and the upper capacitance electrode 105B comes into contact with the insulating film 106. Thus, a first capacitance is formed between the upper capacitance electrode 105B and the lower capacitance electrode 105A. In addition, in a state where the electrostatic-driving-type actuator 101 is not being driven, a gap is formed between the upper capacitance electrode 105B and the insulating film 106. Consequently, a second capacitance, which has a smaller capacitance value than the first capacitance, is formed between the upper capacitance electrode 105B and the lower capacitance electrode 105A. Thus, the electrostatic-driving-type actuator 101 functions as a variable capacitance element.
A phenomenon called a sticking phenomenon sometimes occurs in this type of electrostatic-driving-type actuator. In more detail, the upper driving electrode also comes into contact with the insulating film when the upper capacitance electrode comes into contact with the insulating film when the electrostatic-driving-type actuator is driven. At such a time, charge may enter the insulating film due to there being a potential difference between the upper driving electrode and the lower driving electrode, the charge then becomes trapped in the insulating film, and as a result the insulating film is charged up (electrified). Then, even when application of the driving voltage is stopped, the upper driving electrode will still be drawn toward the insulating film due to the insulating film having been charged up. A phenomenon in which the elastic member does not move from a state of being drawn toward the semiconductor substrate for such a reason is called a sticking phenomenon, and when the sticking phenomenon occurs there is a problem in that the electrostatic-driving-type actuator can no longer be controlled.
Consequently, in the above-described electrostatic-driving-type actuator 101, when driving is performed, switching is performed between a state in which the lower driving electrode 104A is connected to ground while a driving voltage is applied to the upper driving electrode 104B and a state in which the upper driving electrode 104B is connected to ground while a driving voltage is applied to the lower driving electrode 104A (hereafter, this type of driving method is referred to as bipolar driving). Charging up of the insulating film 106 can be eliminated and occurrence of the sticking phenomenon can be prevented by performing such bipolar driving.